Anodising of aluminium and its alloys

ABSTRACT

A METHOD OF ANODISING ALUMINUM AND ITS ALLOYS IN AN ELECTROLYTE COMPRISING 3% TO 30% BY VOLUME OF AN AQUEOUS SOLUTION OF SULPHURIC ACID COMPRISING ADDING BETWEEN 0.1% AND 2% BY VOLUME OF NITRIC ACID TO THE ELECROLYTE BEFORE OR DURING THE CARRYING OUT OF THE ANODISING OPERATION MAINTAINING THE ELECTROLYTE AT A TEMPERATURE BETWEEN 0*C. AND 25*C. AND CARRYING OUT THE ANODISING OPERATION AT AN APPLIED VOLTAGE BETWEEN 10 AND 60 AND AT A CURRENT DENSITY BETWEEN 60 AND 120 AMPERES PER SQUARE FOOT.

United States Patent Oifice US. Cl. 204--58 1 Claim ABSTRACT OF THEDISCLOSURE A method of anodising aluminium and its alloys in anelectrolyte comprising 3% to 30% by volume of an aqueous solution ofsulphuric acid comprising adding between 0.1% and 2% by, volume ofnitric acid to the electrolyte before or during the carrying out of theanodising operation maintaining the electrolyte at a temperature betweenC. and 25 C. and carrying out the anodising operation at an appliedvoltage between 10 and 60 and at a current density between 60 and 120amperes per square foot.

This invention relates to a method of anodising aluminium and itsalloys, for example, in the form of wrought or cast articles.

The anodising of aluminium. and its alloys in electrolytes includingsulphuric acid solutions is well known. The concentration of sulphuricacid employed ranges from 5% by volume up to over 20% by volume and the.anodic coatings produced range in thickness from 2 microns or less upto over 25 microns, the current densities ranging from to 60 amperes persquare foot and the applied voltage varying between 12 and 20 volts. Thetemperature of the electrolyte must be maintained at between 15 C. and25 C. for optimum results.

Very hard anodic coatings have also been produced on aluminium alloys attemperatures below 5 C., usually 0 C., but the voltages required toproduce the current densities of 10 to 60 amperes per square foot arehigher than those just referred to and may rise to as high as 100 voltsfor the production of hard coatings of a thickness of over 25 microns onsome alloys. The higher the voltage required to produce a given currentdensity the greater the amount of cooling needed to maintain theelectrolyte at the required temperature.

In addition some aluminium alloys, particularly those containing morethan 1% of copper, nickel or silicon or admixtures of these elements,are very diflicult to anodise or hard anodise in a sulphuric acidelectrolyte using conventional techniques to produce an anodic coatingof sufficient thickness to have a commercial value. Such alloys include,for example, those known under the nomenclature of British Standardsspecifications 1470 and 1490 as LM6, HIS, RR250 and LM4.

It is an object of the present invention to provide an improved methodof anodising aluminium and its alloys whereby the voltage required toproduce a given current density is lowered and the voltage rise duringanodising is reduced, in relation to those of normal processes, yet withthe production of anodic coatings having properties equivalent to thoseof the coatings produced in these normal processes and with considerablesavings in electrical energy and the amount of cooling required tomaintain the required temperature of the electrolyte. A further objectof the invention is to enable acceptable anodic coatings to be formedupon aluminium alloys hitherto found difficult to anodise.

3,563,867 Patented Feb. 16, 1971 According to this invention, a methodof anodising aluminium and its alloys in an electrolyte comprising asulphuric acid solution comprises adding nitric acid to the electrolytebefore or during the carrying out of the anodising operation.

The amount of nitric acid added may vary between 0.1% and 2% both beingpercentages by volume, but the preferred addition is in the region of 1%by volume of the electrolyte. The electrolyte may comprise 3% to 30% byvolume of an aqueous solution of sulphuric acid and preferably comprises7.5% by volume of such solution.

Some examples of the way in which this invention may be carried intopractice will now be described.

Two 1 foot square pieces of an alloy containing 99.0% of aluminium wereanodised in separate baths each containing a 7.5% by volume aqueoussolution of sulphuric acid at 20 0., one bath also containing 1% byvolume of nitric acid (specific gravity 1.42).

The voltages applied and the current densities produced are shown in thefollowing table:

Current density in amperes per square foot Sulphuric Sulphuricacid-nitric acid acid Voltage electrolyte electrolyte It should be notedthat the voltage-current density relationship varies considerably withthe aluminium alloy used.

Film thickness (microns) 7.57 mso'i Volt- Tempera- 7.5% plus 1.0% Alloy1 age ture, 0. H2804 HNO HE9 17 21. 0 30 27 NS3 20 21. 5 27 27 NE 19 21.5 27 27 N84 17 21. 0 20 27 N821 18 21.0 15 17.5

British Standards nomenclature.

It can be appreciated from these results that the addition of nitricacid to the electrolyte resulted in a much higher current density forthe same applied voltage, with a consequent more rapid growth of theanodic coating.

A sample of the alloy HE9 was anodised at a current density of 60amperes per square foot in an electrolyte consisting of a 7.5% by volumesolution of sulphuric acid maintained at 20 C. After one hour thevoltage had risen from 20 to over 50 volts and the average thickness ofthe anodic coating produced was microns.

An exactly similar sample of the alloy HE9 was anodised at the samecurrent density and temperature for one hour in an aqueous electrolytecontaining 7.5%

3 by volume of H 50; and 1.0% by volume of HNO (specific gravity 1.42).The voltage only rose from 20 to 22 volts and the average thickness ofthe anodic coating produced was 100 microns.

Two samples of the alloy HE9 were separately but similarly anodised toproduce anodic coatings 25 microns thick in respective electrolytes, onecontaining 7.5% by volume of sulphuric acid and the other containing7.5% by volume of sulphuric acid and 1% by volume of nitric acid. Thespecific abrasion resistance of both the coatings, when measured at anair flow rate of 70 litres per minute by method of British Standard1615: 1961, was 2.0 grammes per micron of film thickness.

The sulphuric acid-nitric acid electrolyte is particularly useful forproducing thick hard anodic coatings. In the following table are set outthe film thicknesses obtained on different alloys, at the same voltagesand temperatures (peak voltage of about 60 volts and temperature of 0C.) and a current density of about 36 amperes per square foot, whenusing either a conventional hard anodising electrolyte (17.5% volume H80 and 1.0% by weight oxalic acid) or an electrolyte according to thisinvention (7.5% by volume H 80; and 1.0% by volume HNO Film thickness(microns) Sulphuric Convenacid-nitric tional acid elecelectrolytetrolyte 1 British Standards nomenclature.

Although in the examples given above the electrolyte had a sulphuricacid concentration of 7.5% by volume it has been found that this mayvary between 3% and 30% by volume.

With the use of sulphuric acid-nitric acid electrolytes it' has beenfound possible to produce much thicker anodic coatings than can normallybe produced on certain aluminium alloys, notably those containingappreciable amounts of copper or silicon, which are frequently requiredto be hard anodised. Moreover, the use of lower voltages made possibleby the choice of this electrolyte considerably facilitates thecontinuous anodic oxidation of aluminium wire and strip. In addition thepresent invention enables anodic coatings of acceptable commercialthickness to be produced on aluminium alloys hitherto regarded asdilficult, such as those containing more than 1% of copper, nickel orsilicon or admixtures of these elements.

Although as described above it is assumed that the nitric acid is addedto the sulphuric acid solution before anodising commences it will beunderstood that nitric acid could be added to a sulphuric acidelectrolyte during the anodising operation.

We claim:

1. A method of anodising aluminium and its alloys in an electrolytecomprising 3% to 30% by volume of an aqueous solution of sulphuric acidcomprising adding between 0.1% and 2% by volume of nitric acid to theelectrolyte before or during the carrying out of the anodising operationmaintaining the electrolyte at a temperature between 0 C. and 25 C. andcarrying out the anodising operation at an applied voltage between 10and and at a current density between 60 and amperes per square foot.

References Cited UNITED STATES PATENTS 2,231,086 2/1941 Mi'lller 204--58FOREIGN PATENTS 396,743 8/1933 Great Britain.

JOHN H. MACK, Primary Examiner 45 R. L. ANDREWS, Assistant Examiner

